The next generation of nano- and microsystems will likely require increasingly small and complex structures made of versatile materials. Growth of the commercial MEMS industry has fostered many manufacturing innovations towards extreme miniaturization, including immersion, deep UV, and nanoimprint lithography, and including versatile patterning techniques such as SU-8 processing, deep reactive ion etching of silicon, and soft lithography using PDMS. However, it remains challenging to create freeform (3D) shapes at the nano- and micro-scales. For example, very few processes allow generation of non-planar, curved, or re-entrant geometries. This impedes the development of such things as photonic crystals, biomimetic microsystems, scaffolds for tissue engineering, high luminosity lighting panels, and surfaces having controllable wetting, adhesion, and haptic properties.
Many existing methods of 3D micro- and nanofabrication are based on processing of photosensitive polymers such as SU-8 using, for example, multiple layer, spatially modulated, or inclined exposure techniques. These methods can require multiple sequential alignments and/or may be limited to a maximum inclination angle of about 39°.
Existing methods include serial processes that can create arbitrary 3D forms, including stereolithography, multiphoton lithography, and focused ion beam (FIB) machining. These methods do not particularly lend themselves to batch manufacturing processes, due to, for instance, a tradeoff between resolution and throughput, and may be practically limited to fabricating master templates, forming individual microstructures, or making small batches.
Holographic lithography is another method that can be used to create arrays of 3D microstructures in parallel. But microstructure geometry is determined by the interference of laser beams, limiting the available geometries to periodic patterns.
Further, most of these known fabrication techniques are useful with a limited number of select polymers and are not useful with more structural materials such as ceramics and metals. This can limit the utility of the structures generated by these techniques. For example, SU-8 has a glass transition temperature of only 200° C. and a Young's modulus (E) of about 5 GPa, thereby limiting the environment and conditions in which it can be useful.
Growth of vertically-aligned forests or arrays of nanostructures such as carbon nanotubes (CNTs) or silicon nanowires (SiNWs) is a viable means of self-assembling nanostructures into larger-scale structures with dimensions ranging from the sub-micrometer to millimeter scale. However, despite the exceptional functional properties of individual nanostructures such as CNTs, the bulk properties of such forests are typically poor due to the low density of nanostructure growth, typically about 1-5% of that of an ideal tightly packed configuration. This can be a significant limitation for practical applications, because the spacing among the CNTs can dominate the bulk properties of the forest. For example, the elastic modulus of a CNT forest with 1% packing density is in the range of only tens of MPa, while the expected elastic modulus for individual CNTs is in the range of hundreds of GPa. This can render nanostructure forests inadequate for many nano- and micro-device applications. It can also limit compatibility with post-processing techniques that can require greater structural stability than is available with low density nanostructure arrays. While improvements in the as-grown density of CNT forests are possible, it is practically limited by the required spacing between the catalyst particles on the substrate. It is even more difficult to increase the density of smaller diameter CNTs because smaller catalyst nanoparticles are more mobile on the substrate. Increasing the packing density in CNT arrays can improve their bulk properties, thereby making CNTs more useful in practical applications.
One method of creating 3D microstructures from nanostructures includes confining nanostructure growth within etched silicon cavities such as inverted microchannels clamped to the growth substrate. In this manner, nanostructures can be “grow-molded” into convex 3D shapes, such as those illustrated in FIG. 1. However, the compressive forces exerted by the cavity can destroy nanostructure alignment and introduce structural defects during formation. Such a grow-mold process limits the geometry of the final structure and does not accommodate thin walls or re-entrant curve shapes.
One method of bulk densification of nanostructure arrays using capillary forces includes immersing the arrays in a liquid. This method generally includes immersing or dipping nanostructure arrays attached to their growth substrate into a liquid and withdrawing them from the liquid. This method has been shown to enable only bulk contraction or unidirectional manipulation of the arrays and has other limitations, often resulting in the liquid menisci bridging multiple adjacent nanostructure arrays on the substrate during immersion and/or withdrawal from the liquid. FIG. 2(a) shows a set of patterned nanostructure arrays on a growth substrate, with FIG. 2(b) depicting a possible result of using a liquid immersion method to densify the nanostructure arrays of FIG. 2(a). While the resulting nanostructure arrays may be densified, the capillary forces induced by the bulk liquid can result in the arrays collapsing to positions adjacent to the substrate, generally being toppled in the direction of gravity as the substrate is withdrawn from the liquid. An arrow indicating the direction of immersion and withdrawal from the liquid is shown in FIG. 2(b), and toppling of the arrays, generally in the opposite direction, is illustrated.
These methods of shaping using molds to confine growth and liquid immersion to densify nanostructure arrays are inherently limited in their ability to generate the variety of shapes and bulk properties for which the arrays are potentially useful. Synthetic replication of many natural processes requires more complex shaping ability. For example, nature uses self-assembly to create hierarchical freeform geometries such as tissues and skeletal structures, that have inherent anisotropy that creates directional and responsive properties. However, synthetic self-assembly typically lacks deterministic control of shape at a length scale far exceeding that of the constituent building block. Nevertheless, self-assembly offers promise to complement top-down fabrication by creating hierarchical structures made of diverse components including block copolymers, DNA strands, nanocrystals, nanowires, and nanotubes. In particular, surface tension is useful for manipulating small structures, and has been utilized to fold thin film, to interlock micro-components, and to create tightly packed arrangements of nanoparticles and microspheres or to aggregate wet hair, microfibers, and polymer pillars.